1. Field of the Invention
The present invention relates to a solid state image device and a method of producing the same, and more particularly to a solid state image device which is driven at a high speed for adapting to HDTV (High Definition TV) systems or the like, and a method of producing the same.
2. Description of the Related Art
An experimental TV program for a HDTV system has been broadcast in recent years, and there has been an increase in demand for promptly commercializing the devices for HDTV systems. An HDTV camera is one of those devices.
Since HDTV cameras commercially available today use an image tube, they have the problems due to these image tubes. Specifically, the image tube tends to break when it is subjected to a large external force, or when it is vibrated. It is also difficult to produce a smaller image tube. Thus, the development of an HDTV camera employing a solid state image device which can overcome these problems is desirable. Because conventional solid state image devices have the following problems, they are not suitable for HDTV cameras.
FIG. 9 shows a partial plan view of a conventional solid state image device 100. In the solid state image device 100, photodiodes 101 for converting light into electric charges are formed in a matrix pattern along x and y directions in a semiconductor substrate 102. Vertical transfer regions 103 extending along the y direction are formed between the photodiodes 101 in the semiconductor substrate 102. The regions where the photodiode 101 and the vertical transfer region 103 are provided are referred to as a photosensitive portion and a vertical transfer portion, respectively. The region between two adjacent photosensitive portions along the y direction is referred to as a clip portion.
A plurality of first Gate electrodes 104 shown by dotted lines and a plurality of second gate electrodes 105 shown by solid lines, both in a comb shape, are provided between the photodiodes 101 over the semiconductor substrate 102.
Each of the first gate electrodes 104 includes transfer electrode portions 106 provided on the vertical transfer regions 103 through an insulating film, and clip electrode portions 107 for connecting the vertical transfer electrode portions 106 to each other in the x direction. Similarly, each of the second Gate electrodes 105 includes vertical transfer electrode portions 108 provided on the vertical transfer regions 103 through an insulating film, and clip electrode portions 109 for connecting the vertical transfer electrode portions 108 to each other in the x direction.
After electric charges are accumulated in photodiodes 101, each of the electric charges is transferred to the vertical transfer region 103 as indicated with an arrow 110 by a driving pulse voltage applied to the second gate electrodes 105. Then, the electric charge is transferred in the vertical transfer region 103 in the direction indicated by an arrow 111 by driving pulse voltages applied to the first gate electrodes 104 and the second gate electrodes 105.
To apply the solid state image device 100 for an HDTV camera, it is necessary that the solid state image device 100 has a large number of pixels, such as 1.3 or 2.0 million, and is driven at a high speed. Specifically, the electric charges accumulated in the photodiodes 101 need to be transferred at a high speed by driving pulse voltages applied to the first and second gate electrodes 104 and 105.
However, since the size of the solid state image device 100 increases as the number of the pixels increases, the first and second electrodes 104 and 105 become longer, the effect of the resistance of the gate electrodes themselves become significant. Thus, since the middle portions of the first and second electrodes 104 and 105 are most distant from both ends of each electrode, the effect of the resistances of the gate electrodes to which driving pulse voltages are applied is significant in these portions. Therefore, the pulse width of a driving pulse voltage decreases in the middle portions of the gate electrodes, and the maximum amount of electric charge transfer is disadvantageously reduced.
Conventional gate electrodes are made integrally by forming polysilicon films, each of which has a uniform film thickness, on a semiconductor substrate, and etching them into a predetermined shape. More specifically, after the deposition of a polysilicon film 121 having a thickness of about 450 nm on an insulating film 120 as shown in FIG. 10A, a resist pattern 122 is formed on the polysilicon film 121 as shown in FIG. 10B. Then, the polysilicon film 121 is etched using the resist pattern 122 as a mask, and the first gate electrodes 104 are formed by removing the resist pattern 122. As a result, the first gate electrodes 104 of the conventional art having a thickness of 450 nm is formed on the entire region which has been subjected to the formation steps. Since the polysilicon has a high resistance, the problem described above is significant.